The invention relates to an electromechanically actuatable brake.
From British Patent GB 2 190 441, one such electromechanically actuatable brake is known with a two-part actuating device, namely a first part for overcoming the air clearance and a second part for pressing a brake lining against a brake body. Both parts of the actuating device have a separate spindle drive, drivable by its own electric actuating motor. The two parts of the actuating device can be driven simultaneously or successively to execute a brake actuation, by putting the two motors into operation. Both parts of the actuating device are joined together by a blocking bracing means embodied as a pair of levers. The pair of levers is pivotably supported about a stationary shaft. The first part of the actuating device engages the pair of levers with slight spacing from the shaft, while the second part of the actuating device engages the pair of levers with major spacing from the shaft. Because of this arrangement, the first part of the actuating device, which acts directly on the brake lining, is capable of rapidly overcoming the air clearance; via the pair of levers and the first part of the actuating device, the second part of the actuating device can generate a high contact-pressure force on the brake lining. Both motors of the actuating device can be equipped with a brake. Thus, unintended adjustment of the brake during braking operations can be avoided with a constant braking force while the motors are turned off.
The known brake is intended particularly for use in railroad vehicles. There the requisite installation space for the two parts of the actuating device and the relatively large-volume for the pair of levers is available in the known brake device. But because of its weight and volume, the brake would be unsuited to disposition in the bowl of a wheel rim of a road vehicle. Moreover, because of the two motors, the brake is expensive and requires increased expense for control. Thus, the bracing means is blocked only when, in successive drive of the two parts of the actuating device, the part serving to press the brake lining is not driven, or in other words only whenever only the air clearance is overcome. If both parts of the actuating device are driven simultaneously, conversely, the bracing means is not blocked. The bracing means can be released by swiveling the pair of levers about the fixed shaft. Pivoting the pair of levers is done by driving the motor of the part of the actuating device used to press the brake lining. If one or both motors of the actuating device fail, problems can arise in releasing the actuating brake.
From GB 2 190 441, a brake is also known for pressing a brake lining against a rotating brake body (brake disk, brake disk or the like), which has an actuating device with a threaded roller drive, which is drivable by an electric motor. Both tightening and releasing the known brake are accomplished with the electric motor. To prevent residual braking moments caused by hysteresis of the actuating unit from acting on the brake body in the event of a malfunction, such as failure of an electronic control system of the brake during braking, a preferably spiral restoring spring is provided in one embodiment of the known brake; this spring engages the actuating unit and drives the actuating unit, together with the electric motor, to rotate in the release direction so that the brake lining is lifted from the brake body.
This brake has the disadvantage that when brake pressure is built up, the force of the restoring spring must additionally be overcome, and the electric motor must therefore be dimensioned correspondingly larger and supplied with a higher current. In addition, there is a dynamic loss, and a loss of efficiency.
Another disadvantage is that to keep a built-up brake force constant, the electric motor must be supplied with such a high current that it keeps the brake lining pressed against the brake body with a constant contact pressure, counter to the force of the restoring spring, which entails thermal problems. Another factor is that the known brake cannot be used as a parking brake, because it releases when it has no current. Another disadvantage is that the brake cannot be released if the threaded roller drive is jammed.
Another disadvantage is that an air clearance, that is, a spacing between the brake lining and the brake body when the brake is released, increases with increasing wear of the brake linings. As a result, on the one hand a positioning travel of the brake lining until the brake lining contacts the brake body becomes greater and accordingly it takes longer until the brake grabs. The dynamic loss is additionally increased. Furthermore, the force necessary to overcome the force of the restoring spring increases, because the restoring spring is deformed more markedly. The energy that has to be brought to bear by the electric motor of the actuating device to deform the restoring spring is equivalent to the product of the deformation travel and deformation force; thus as the air clearance increases, this energy increases at least quadratically, which quite severely worsens the efficiency of the brake when the brake linings become worn.